The Board of Regents of the University of Nebraska acknowledges that some funding for the research leg to this application was provided by the United States Government.
The present invention relates to detection of neutrons More specifically, the present invention relates to a method and device for the efficient detection of neutrons that employs a boron-rich semiconductor as an electrically active part of the detection device.
Neutron scatting is an important research method to determine the structure of solids and liquids. It is used to understand the forces that act between the atoms in these systems and to determine the magnetic behavior of materials as well. The research and practical applications cover a broad range of areas, from the basic properties of materials to studies of engineering and medical applications.
There are essentially only four elements suitable for forming solid state semiconductor neutron detectorsxe2x80x94boron (B), cadmium (Cd), gadolinium (Gd) and lithium (Li). Lithium semiconductor materials exist (LiInS2, LiInSe2 and LiZnP) but are difficult to reliably fabricate into devices and are very difficult materials with which to work Gadolinium conversion layer based silicon (Si) diodes have been fabricated and proposed for neutron detection, but are not particularly stable. Cadmium zinc telluride has been shown to yield thermal neutron detection and the cadmium neutron capture cross section is high, but the neutron capture produces such high energy gamma rays (over 0.5 MeV) that the detectors would have to be large in order to detect these gammas efficiently.
Use of boron with neutron detectors is known both in the scintillator, the gas and the conversion layer varieties. Boron phosphide (BP) heterojunction diodes with silicon were successfully tested as alpha radiation detectors, but failed to work as neutron detectors. Boron carbide (B4C) was successfully used as a neutron detector based upon resistivity changes resulting from increased lithium doping, as were (111) BP wafers. The lithium production in the boron carbide was a result of the following nuclear reactions:
10B+nxe2x86x927Li (1.01 MeV)+4He (1.78 MeV)
10B+nxe2x86x927Li (0.83 MeV)+4He (1.47 MeV)+xcex3(0.48 MeV)
Boron has also been considered as a coating to a silicon diode and a GaAs diode but the maximum efficiency is low (less than 5%).
Existing gas and liquid neutron detectors are much larger and less rugged than solid-state ones could be. However, existing solid state neutron detectors also suffer serious limitations. For example, known boron doped semiconductors are only a few percent efficient because they contain relatively little boron. Gadolinium, lithium and hydrocarbon conversion layers are all adversely affected by corrosion and high temperatures.
Furthermore, known conversion layer devices have low efficiencies, unless multiply stacked, because the range of the reaction products in the material of the conversion layer is generally considerably less than the thickness required for stopping thermal neutrons. Gadolinium conversion layers are an exceptionxe2x80x94but the neutronxe2x80x94gadolinium reaction results in conversion electrons of relatively low energy (70 keV) compared with the reaction products in the case of neutron capture by boron 10. Cadmium zinc telluride has been shown to yield thermal neutron detection, but the neutron capture produces such high energy gamma rays (over 0.5 MeV) that the detectors must be large to detect these gammas efficiently. Scintillator combinations with photomultipliers or intensified cameras are bulky and heavy and, except for neutron-detecting scintillating fibers coupled optically to a remote photomultiplier or camera, are intolerant of high temperatures.
Boron and boron compounds, including boron carbide, are also used in neutron absorbing shielding purposes in nuclear reactors and other types of neutron radiation environments. For example, boron carbide can be used with shielding, thermal electric power, or detection of neutrons (by means of the resistivity change not by detection of individual neutrons). However, use of boron carbide to detect neutrons where the boron carbide is an electrically active semiconductor is novel.
It is an object of the present invention to provide an inexpensive solid state neutron detector that includes a robust, structurally forgiving boron rich semiconductor.
It is another object of the present invention to provide a boron carbide semiconductor that utilizes its electrical properties as a semiconductor rather than its electrical property of resistance as a means of detecting neutrons or its thermoelectric properties in detecting neutrons.
A still further object of the present invention is to provide a detection device that yields high gain.
A further object of the present invention is to provide a detection device that provides real time response.
A further object of the present invention is to provide a detection device that is capable of detecting single neutrons.
Yet another object of the present invention is to provide a detection device that has low sensitivity to gamma and other radiation.
Still another object of the present invention is to provide a method of detecting neutrons with a detector device having a boron carbide semiconductor.
According to the present invention, the foregoing and other objects are obtained by a detection device having a layer of boron carbide. In the device, the boron carbide layer is an electrically active part of the detection device. The sensing mechanism of the detection device is inherent in the electrically connected, semiconducting boron carbide layer, which provides neutron capture resulting in prompt, innately highly amplified, electrical output signals following interception of neutron(s).
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the practice of the invention. The objects and advantages of the invention ray be realized and attained by means of the forms of instrument and the combinations particularly pointed out in the appended claims.